Such a phenomenon is mainly due to the precipitation of n-paraffins contained in the gas oil.
Obviating such a drawback by adding to the above said hydrocarbons suitable substances, generally of the polymeric type, is known as well.
The additives commonly used for such a purpose are represented by ethylene-vinyl acetate copolymers having suitable molecular weight values and compositions, or, according to Italian patents Nos. 811,873 and 866,519, by ethylene-propylene-(non-conjugated) diene copolymers or terpolymers, prepared with homogeneous-phase catalysts (based on vanadium compounds, and organometallic aluminum compounds).
In U.S. Pat. Nos. 3,374,073 and 3,756,954, as such additives ethylene-propylene-conjugated or non-conjugated diene terpolymers are proposed, which are prepared with homogeneous-phase catalysts, and are subsequently degraded by thermo-oxidation until suitable values of molecular weight are reached.